Tora (the word for 'tiger') is a very powerful, very old beast
named Nagatobimaru.
In the stories passed down through Ushio's family, 500 years ago,
there was an incredible beast, ravenous and ruthless, who spread terror
and fear through the area in the way it feasted on humans. It was one
of Ushio's ancestors who challenged the beast to battle, and eventually
succeeded in pinning the beast to a huge stone, impaling it on the
legendary weapon called the 'Bakemono Yari' ("The Beast Spear")
This was Nagatobimaru.

Having defeated the beast, Ushio's ancestor retired from fighting,
becoming a monk and erecting a temple around the spot where the beast
lay trapped on the stone. For centuries, the beast lay in the basement
of the temple, trapped and sealed away, its evil energy growing...

One day, by accident, Aotsuki Ushio finds him in the basement, and the
beast persuades him to remove the spear, freeing him, but then finds
himself in a bit of a predicament: Ushio, in wielding the spear like
his ancestor, becomes the same beast-slaying warrior that trapped him
500 years ago--and one that prefers keeping him under control.

An uneasy alliance forms between Ushio and 'Tora' (as Ushio so names
him a bit later), with each of them secretly muttering threats to
destroy the other, yet never seeming to ever go through with it...
While indeed a beast, and perhaps a bit gruff and selfish, Tora always
seems to find a selfish reason to do the 'good' thing, as if his
selfishness is merely a cover so that he doesn't seem weak and soft.

Tora is indeed a very powerful, very vicious supernatutal creature,
endowed with beastly strength, incredible powers of regeneration,
and his dreaded mystical lightning attacks. Later on, he even shows
further abilities, controlling his fur and hair, and even directing
his limbs to strike opponents even after they are severed from him.
Another supernatural creature (a spirit by the name of Umizato) calls
Tora "Lord Nagatobimaru", and seemingly for good reason!